1. Field of the Invention
This invention relates to optical couplers. More particularly, this invention relates to cylindrical lenses to couple laser energy into linearly configured optical fibers.
2. Description of the Related Art
Prior apparatus for coupling laser energy into a bundle of optical fibers utilized one or more lenses to sharply focus the laser energy into a focal point. The bundle was arranged with input ends of the fibers in a circular or hexagonal pattern. The circularly arranged fiber ends were placed within the laser beam at a distance from the focal point such that the cross sectional diameter of the laser beam substantially matched the diameter of the circular pattern of optical fibers
This arrangement, however, suffers from at least two distinct problems. First, the coupling system produced an unevenly distributed laser energy profile at the incident ends of the optical fibers such that the respective optical fibers transmitted the energy in nonuniform amounts. In particular, launching a Gaussian laser beam into a circular or hexagonally shaped optical fiber bundle will result in the fibers nearer the center of the circular or hexagonal configuration transmitting greater amounts of energy than the fibers nearer the periphery of the circular or hexagonal configuration. Second, the naturally rectangular shape of the laser beam exiting the laser was not conducive to "rounding" into the circular or hexagonal configuration of the optical fibers without significant energy loss.
These problems of uneven energy distribution in the transmission fibers and uncoordinated beam shapes between the laser and the fibers resulted in inefficient energy coupling between the laser and the optical fibers.
The problem of uneven distribution of laser energy within the respective optical fibers causes the total power transmission capability to be dictated by the peak power rating of the central fibers, which will be carrying the most energy. Thus, the total power coupled into the fibers must be decreased to bring the maximum power coupled into the individual central fibers to within their energy damage thresholds. Meanwhile, the peripheral fibers receiving the lower energy concentration are not being used to their peak energy transmission ability due to the caution which must be observed in preventing destruction of the central fibers in the higher energy concentration region. Therefore, to prevent damage to the central fibers, total coupled energy must be reduced even though the peripheral fibers are not used to their peak energy transmission ability.
Several remedies to the problem of uneven energy distribution in the optical fiber configuration have been implemented. Known methods to more uniformly distribute the beam energy along the entire optical fiber profile include using beam homogenizers, aspheric lenses, or afocal doublets. The beam homogenizer folds back the side lobes of an incident laser beam onto the central portion of the beam to create a homogenized beam of more uniform intensity across its energy profile. One known method of homogenizing an energy beam is by using symmetrical mirror pairs located along the axis of the beam. The first set of mirrors separates the sides lobes of the beam from the center of the beam and the second set of mirrors folds the beam, imaging each side lobe to the opposite side of the central axis from where it originated. The resultant superimposed beam is more uniformly distributed across the profile than the original beam profile.
Other known methods of unifying the beam profile include the use of aspheric lenses. An aspheric plate, usually having a conic surface of revolution about the lens axis, deviates an incoming laser beam so the intensity of the beam becomes uniform across the beam profile. A second plate then deviates the waves to eliminate spherical aberration without modifying the uniformly distributed energy profile. The result is an unaberrated, substantially uniformly distributed laser energy profile to be coupled into the circular or hexagonal optical fiber configuration.
Afocal doublets are also employed in known methods of unifying the energy distribution of a laser beam. A doublet is a lens comprising closely spaced positive and negative elements. In the typical arrangement, the lenses are bent to introduce a large amount of spherical aberration. The laser beam is then passed through the first element, with the paraxial rays being essentially unaffected by the spherical aberration while the other rays will see a large amount of uncorrected spherical aberration. Thus, the paraxial rays will remain collimated in the space between the first and second elements while the peripheral rays seeing the spherical aberration will converge in the space between the lenses. By the time the rays meet the second lens, the peripheral rays have bunched together as they converge such that the energy at the periphery of the energy profile is increased. Meanwhile, the centrally located rays have remained collimated such that the energy at the central region of the energy profile is unchanged. The second element has an equal and opposite spherical aberration as the first element to recollimate the energy beam to result in a collimated and substantially uniformly distributed laser beam.
The above methods of alleviating the nonuniformity in the laser energy profile before coupling the laser beam into an optical fiber configuration require no fewer than four elements and also require custom design and custom manufacturing for the particular laser wavelength, energy level, and beam size used. Design and manufacturing costs can be prohibitively high despite the benefits of increased total power coupling capability when the above methods are used to unify the laser energy profile. Accordingly, a simplified coupling system is needed which creates a more uniform laser energy profile using standard optical components and equipment rather than custom components and equipment.
The further problem of manipulating the naturally rectangularly shaped laser energy beam into a circular or hexagonally shaped beam to accommodate the circular or hexagonal optical fiber configuration is not necessarily remedied by the above energy distribution methods. One known method of adjusting the shape of the laser beam to match the circular or hexagonal optical fiber configuration is to pass the naturally rectangular laser beam through a spatial filter prior to encountering the optical fiber ends. The spatial filter will round the laser beam by blocking the outer portions of the rectangular beam, unfortunately resulting in a significant amount of energy loss before the laser beam even encounters the optical fiber ends.
Accordingly, a coupling system is needed which efficiently matches the shape of the laser beam with the shape of the optical fiber configuration.